The problem for DL is there is no plane for them to swap the 717s leases with other than the 737Max.
They're clearly committed to the A359 and A330 for long haul and they just refreshed the 777s so no need to top-up there. With demand dropping precipitously, any need for the 777X just disappeared and with Boeing's financial woes, you can put all talk of the NMA to bed for a while. I guess they could swap the 717s for some 787s but I'm sure their models show domestic travel coming back before international travel which makes a 737Max far more attractive than the 787 and again the overlap with the A359 and A339 is too high to be worth introducing a new fleet type.
That leaves DL with either holding onto the 717 leases or trying to make this wild deal. We also know that DL is shrewd at negotiating and bought the A220 when it was basically on its death bed.
The fact that Boeing is the lessor here makes this especially interesting because it effectively blocks Airbus from making a counter offer unless Airbus offers to cover the cost of the 717 leases